1. Field of the Invention
The present invention relates to an oscillation control device for controlling the frequency of an oscillator and to a synchronization system for synchronizing the frequency of the oscillator with the frequency of an oscillator located at a remote site.
2. Description of Related Art
In earlier developments, there is a remote calibration system in which a reference value for enabling the precision measurement for a measurement instrument is supplied from a remote site (See JP-2004-326671A: Document 1). “Calibration” in the remote calibration system means a series of operations for defining the relationship between the value indicated by an instrument or a measurement system or the value represented by a material measure or by a reference material and the values realized by the standards.
As an example, in the remote calibration system, the difference between the frequency b of an oscillator under calibration which is located at a site B and the frequency f of the GPS signal transmitted from a GPS (Global Positioning System) satellite (i.e. b−f) is measured, and at the same measurement time, the difference between the reference frequency a of the reference oscillator (e.g. a standard oscillator of the National Metrology Institute) located at the site A and the frequency f of the GPS signal transmitted from the GPS satellite (i.e. a−f) is measured. Then, the difference between the data obtained by both measurements, that is, {(b−f)−(a−f)=(b−a)} is obtained through the communication network such as the Internet. As this difference frequency (b−a) represents the deviation of the frequency b of the oscillator under calibration from the reference frequency a of the reference oscillator, the oscillator located at the site B comes to be calibrated.
In Data Sheet “58503B GPS Time and Frequency Reference Receiver”, by Symmetricom company: Document 2, an oscillator which is capable of synchronizing with the GPS signal has been proposed. FIG. 3 shows the principal part of the arrangement of an oscillator 200 shown in the Document 2. In the oscillator 200, a signal synchronizing with the GPS time (hereinafter called “synchronized signal”) is produced when the GPS signal is received by a GPS receiving antenna 201, and is outputted to a frequency control section 203. In the frequency control section 203, the frequencies of the synchronized signal inputted thereto and of the oscillation signal outputted from a frequency oscillation section 204 (i.e. the oscillation signal of the oscillator 200) are compared with each other, and feedback is applied to the frequency oscillation section 204 so that the difference between both frequencies will be zero. In this manner and operation, the oscillation frequency of the oscillator 200 can be synchronized with the GPS time.
In the remote calibration system disclosed in Document 1, the frequency of the oscillator under calibration, which is located at the site B remote from the site A where the reference oscillator is located, still includes a deviation from the frequency of the national standard. Therefore, in the case that other measurement instruments are calibrated or adjusted by using the frequency of the oscillator at the site B, it is required to do based on the calibration certificate sent from the National Metrology Institute (any form of the certificate, including mailed document or electronically converted data sent through the communication network). As it usually takes several days to several weeks in order to get the calibration certificate, this system has a drawback in which such calibration or adjustment can not be performed in real time.
On the other hand, in the oscillator disclosed in Document 2, the frequency equivalent to the accurate frequency provided by the GPS satellite can be obtained in real time. However, because there exists a deviation between the frequency obtained from the GPS and the frequency provided by the standard oscillator of the National Metrology Institute and the scale thereof is not measured, the traceability can not be secured. In order to secure the traceability, it would be necessary to implement either the method similar to one that is disclosed in the Document 1 or to carry on (or to transport) the oscillator to the National Metrology Institute.
As mentioned above, a conventional oscillator has a drawback in which it cannot synchronize its own frequency in real time with the frequency of the standard oscillator located at the remote site.